Fully dense flat products of titanium alloys, titanium metal matrix composites, and titanium aluminides are of particularly great interest in the aerospace, automotive, sporting goods, and other industries due to their excellent strength-to-density ratio, stiffness, strength and fatigue related properties, and high temperature and corrosion resistance. But the manufacturing of titanium-based strips, plates, sheets, or foils is characterized by high production costs of multiple rolling/annealing operations that are caused by relatively high hardness and low ductility of titanium alloys, especially, titanium matrix composites and titanium aluminides. Multiple rolling/annealing cycles create textured materials whose mechanical properties are not uniform in transvers and longitudinal directions. Besides expensive processing of titanium alloys, it is very difficult to manufacture the reinforced titanium-based materials, as well as composite multilayer structures using the conventional technologies. In some applications, it is desirable to increase stiffness of the titanium alloys by reinforcing them with various hard particles. The reinforcing components should be thoroughly and uniformly dispersed in the volume of the matrix alloy to achieve the maximum mechanical properties of the composite strips. It is extremely difficult to manufacture such high-performing composite flat products by conventional wrought metallurgy. Another application requires production of the titanium composite structures having high fracture toughness core layer with high-temperature capability of external layers, such as a TiAl/Ti6Al-4V/TiAl composite. Manufacture of these composite structures is also very expensive by the conventional wrought metallurgy techniques.
The direct powder rolling process, among other competitive methods, has the potential of becoming a cost-effective method of manufacturing strip products from a variety of powder metallurgy alloys, multilayer structures, and composite materials. It is possible to produce titanium alloy strips by an economically attractive process using direct powder rolling at room temperature in air and subsequent sintering in a protective atmosphere.
Direct powder rolling of blended elemental titanium alloys promises a solution of both economical and quality problems that can provide the near full density flat product material produced by this cost effective manufacturing process.
Despite more than fifty years of experience in industrial applications for making different metals and alloys, conventional direct powder rolling processes had not been used in the manufacture of titanium flat products. For example, methods for manufacturing strips from blended elemental powders disclosed in the U.S. Pat Nos. 4,602,954 and 4,617,054 cannot provide 100% density strips due to a presence of residues of organic binders that do not allow to achieve an effective densification by compaction during cold rolling of green strip, moreover, evaporation of binders creates the voids which cannot be healed during sintering and which form so-called gaseous porosity.
Another sources of porosity in sintered strips are the diffusion voids resulted from the mutual diffusion interaction between the titanium base particles and the particles of alloying elements at the sintering temperature. The larger the particles of alloying elements, the bigger the voids developed during sintering. No one of the methods known from the prior art can avoid this type of porosity in final products.
Conventional technology of direct powder rolling of blended titanium alloys always is characterized by both types of porosity, gaseous and diffusion. Increasing the compression forces during the rolling of green strips results in cracking of the rolled metal due to difference in mechanical properties of alloying elements in the blends. Therefore, such methods as described in the U.S. Pat. No. 4,108,651 which are effective for some metal powders are not effective for direct powder rolling the blended elemental titanium alloys.
Thus, all prior art methods of fabricating dense strip products from various metal powders by direct powder rolling and sintering have considerable problems if titanium alloy powders are being used. Technological drawbacks associated with low ductility and diffusion and gaseous porosities make the direct powder rolling process unacceptable when strips, plated, and foils are being rolled from titanium alloy powders because the finished flat products are not fully dense and insufficient mechanical properties make these products unacceptable for industrial applications. Therefore, the low-cost direct powder rolling process is not currently being used in the titanium industry.
The process of this invention offers the advantages over the conventional powder metallurgy techniques. Furthermore, the method overcomes the above mentioned limitations associated with the prior art which prevented the achievements of fully dense titanium alloy strips, plates, sheets, and foils manufactured by direct rolling of blended elemental powders.